Application of Magnetic Suspension and Balance Systems to Hypersonic Wind Tunnel Testing

Wind tunnel model sting and shroud interference effects can be significant especially when testing small slender models which are required to simulate higher Knudsen number flows. Indeed, the presence of a sting of any size complicates all flowfield calculations since the model/sting geometry must also be numerically modeled. Thus, in an attempt to generate reliable low density aerothermodynamic data on test models in flows closer to the flight test ideal, the University of Oxford Low Density Wind Tunnel was recomissioned for an extensive series of experiments in which a magnetic suspension and balance system was used to obtain sting and shroud free model data. Test data were obtained over wide ranges of freestream Mach (M= 6 - 9) Knudsen numbers (.001-0.3) on a wide variety of half angle cones at angle of attack, axially aligned cylinders of varying aspect ratio, and a model of the NASA Aerobrake vehicle. In this paper, the critical components and functions of the magnetic suspension and balance system along with the levitation and test measurement procedures will be described. Techniques used to achieve model stability through the active control of lift, pitch and drag, and lateral stability, including suitable shaping of the magnetic field, and strategic placement of ferromagnetic material within the test section will be discussed. Coil placement, coil current control loop, and the interactive magnetic field and optical control procedures required for successful levitation will also be described. Details of the optical model position detection and feedback system which were developed for both conical and axisymmetric model levitation and testing will also be presented along with the procedures developed for drag calibration and measurement. Some of the data that were acquired during the program will also be presented and they include: Aerodynamic heat transfer and recovery factor measurements on slender cones, and the drag data obtained for the NASA Aerobrake vehicle.